Fix failure to run executable targets on Windows

[cerfical]

Fixes #228

[tristan957]

Could you describe how this fixes the issue? Thanks for your contribution

[cerfical]

Well, the code below parses the environment file meson-vscode.env:

vscode-meson/src/utils.ts

Lines 169 to 178 in 3f4ee1e

	_envDict = {};
	const data = await fs.promises.readFile(envfile);
	for (const i of data.toString().split(/\r?\n/)) {
	// Poor man's i.split("=", 1), JS won't return part after first equal sign.
	// Value is quoted, remove first and last " char and also possible \r ending.
	const index = i.indexOf("=");
	const key = i.substring(0, index);
	const value = i.slice(index + 2, -1);
	_envDict[key] = value;
	}

I haven't looked into how exactly the file gets its contents, but I guess that it must contain some kind of environment variables defined as <key>=<value> pairs. Later, the environment is used to initialize a new process responsible for a run task:

vscode-meson/src/tasks.ts

Lines 47 to 50 in 3f4ee1e

	new vscode.ProcessExecution(t.filename[0], {
	cwd: workspaceState.get<string>("mesonbuild.sourceDir")!,
	env: getEnvDict(),
	}),

The problem occurs because envFile can contain an empty line (specifically at the end of file), which causes an empty key with an empty value to be inserted into the resulting dict. The implementation then calls a system API to create a process and fails. So for a simple solution, we can just ignore empty lines by filter()'ing them out. Or we could redesign some things, and that requires extra work for such a small issue, so...